1. Field of the Invention
The present invention relates to a vehicular generator motor which acts to generate electric power by receiving torque from an engine and to perform motor operation for imparting torque to the engine.
2. Related Art
One kind of generator motor is described in Japanese Patent Laid-Open No. 202255/1988 and uses MOSFETs as switching devices for controlling the electrical currents flowing through armature coils. Zener diodes are employed to compensate for the voltage-withstanding characteristics of the MOSFETs.
The inventor of this known technique noticed the fact that trade-offs were made between the ON resistance of each MOS transistor fabricated on a silicon substrate and the required withstand voltage of the device. The withstand voltage of the MOS transistor is set lower by adding a power Zener diode. According to this prior art, the resistance during the transistor turns on can be made low value by thinning a voltage-withstanding layer of the device. Thus, it can be considered that the starting characteristics are improved to some extent. However, this known technique suffers from the following problems. When generator action is performed, the added power Zener diodes act as commutating diodes which must handle large electric currents at all times. If the Zener diodes vary among commercial products, energy is concentrated in such a device which has a lower Zener voltage when surges are being absorbed. As a result, the device is easily destroyed. This makes it necessary to set the surge voltage resistance at a high value. Alternatively, it is necessary to check the Zener voltage characteristics of the Zener diodes. Then, they must undergo a sorting step. These greatly increase the cost. Furthermore, separate heat-dissipating fins are required for the Zener diodes. This increases the size of whole apparatus.
More specifically, in the generator motor fabricated in the prior art technique as described above, a high voltage is generated inside the generator motor during generator operation. For example, in the case of a 12 V storage battery, the generated voltage is more than 10 times as high as the battery voltage, i.e., 150 to 300 V. The devices are required to be protected against such a high voltage. This urges the ON resistance of the devices to be set higher than the ON resistance used where the machine performs only motor operation. In the latter case, the devices are required to withstand a voltage that is only several times as high as the power voltage. Especially, when an engine at rest is started, the large starting current is controlled with these high ON resistive devices each having a thick voltage-withstanding layer. This further increases the ON resistance and produces the disadvantage that a considerable portion of the applied battery voltage is lost by the MOSFETs as a voltage drop.